1. Field of the Invention
The present invention generally relates to wireless communications and particularly to locating mobile units in a wireless time division multiple access (TDMA) communication system.
2. Description of the Related Art
The proliferation of mobile units or cell phones and their technology and usage, has suggested various applications for the ability to locate mobile units. These applications include xe2x80x9c911xe2x80x9d calls, tourist and travel information, tracking unauthorized cell phone usage and illegal activities, and locating commercial and government vehicles. The basic cell phone systems can determine the nearest base-station which locates a cell phone only to within 3 to 10 miles. However, FCC Regulations, as dictated in FCC Docket 94-102, require location accuracy of about 400 feet for mobile units (cellular/PCS users) for E-911 (Emergency-911) service by October 1, 2001.
Therefore, a great deal of emphasis has been placed on developing systems that can track the position of mobile units more accurately. One known method for locating mobile units is time difference of arrival (TDOA) which has been used for many years, at least in such applications as the Global Positioning System (GPS). Applying TDOA to cell phones involves measuring the time of arrival of a signal transmitted from a cell phone at a multiplicity of base-stations and comparing the arrival times to determine how long the signal took to reach each base-station. The TDOA algorithm, however, requires that at least three base-stations must be accurately synchronized in time before a mobile unit can be located accurately. This is an expensive and difficult requirement.
A method for computing the position of a mobile unit without synchronizing base-stations is provided. The principles of the present invention use a triangulation scheme where distances between a plurality of base-stations and a mobile unit (herein termed mobile) are calculated by measuring the round-trip delay time (i.e., round-trip delay value) of known symbols in the transmitted signals.
In an illustrative embodiment, upon receiving a 911 emergency call from a mobile, a serving (i.e., primary) base-station transmits a known downlink signal comprising 162 symbols to the mobile. Out of 162 symbols, 14 symbols are sync words and 148 symbols are control and data symbols. The serving base-station also contacts the Master Switching Center (MSC) and requests a list of neighboring base-stations available for hand-off. In response to the inquiry from the serving base-station, the MSC provides a list of neighboring base-stations available for hand-off. Then, the serving base-station selects at least two neighboring base-stations from the list to perform hand-off of the mobile.
In the meantime, the mobile receives the downlink signal transmitted by the serving base-station. The mobile then identifies its unique 14 symbol sync words and then transmits an uplink signal comprising 45 symbols to the serving base-station. This uplink signal is received by the serving base-station, which measures the total round-trip delay value for the downlink and uplink transmissions.
After the total round-trip delay value has been measured, the serving base-station performs a hand-off of the mobile to a first neighboring base-station. The first neighboring base-station receives the hand-off and immediately transmits an analogous downlink signal to the mobile. The mobile receives the downlink signal and in response transmits an analogous uplink signal to the first neighboring base-station. The total round-trip delay value for this second pair of downlink and uplink transmissions is then measured. After measuring the round-trip delay value, the first neighboring base-station performs a hand-off of the mobile to a second neighboring base-station. The second neighboring base-station also transmits an analogous downlink signal to the mobile and in response receives an analogous uplink signal from the mobile. The round-trip delay value for this third pair of downlink and uplink transmissions is then measured. After computing all three round-trip delay values, a triangulation algorithm is used to compute the position of the mobile.
Thus, the principles of the present invention provide a scheme to determine the position of the mobile within the desired range of accuracy without requiring the participating base-stations to be accurately synchronized in time. Thus, the principles of the present invention provide a low-cost solution with the added advantages of utilizing a known downlink signal and a known uplink signal in determining the position of the mobile.
In one embodiment, the present invention is a method for locating a mobile unit in a wireless network having a plurality of base-stations, comprising the steps of (a) receiving at least three round-trip-delay-values, each round-trip-delay-value corresponding to transmission of a downlink signal from one of at least three base-stations to the mobile unit and transmission of an uplink signal, in response to the downlink signal, from the mobile unit to the one base-station; and (b) determining the position of the mobile unit based on the at least three round-trip-delay-values using a triangulation algorithm.
In another embodiment, the present invention is a node of wireless network having a plurality of base-stations, the node configured to locate a mobile unit in the wireless network by (a) receiving at least three round-trip-delay-values, each round-trip-delay-value corresponding to transmission of a downlink signal from one of at least three base-stations to the mobile unit and transmission of an uplink signal, in response to the downlink signal, from the mobile unit to the one base-station; and (b) determining the position of the mobile unit based on the at least three round-trip-delay-values using a triangulation algorithm.